Application of electronic properties of germanium to inhibit n-type or p-type diffusion in silicon

ABSTRACT

A method of inhibiting dopant diffusion in silicon using germanium is provided. Germanium is distributed in substitutional sites in a silicon lattice to form two regions of germanium interposed between a region where dopant is to be introduced and a region from which dopant is to be excluded, the two germanium regions acting as a dopant diffusion barrier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to fabrication of semiconductorintegrated circuits and, in particular, to use of germanium insubstitutional sites in a crystalline silicon lattice as a diffusionbarrier for both n-type and p-type dopant species.

2. Discussion of the Prior Art

The technology of semiconductor integrated circuits is based uponcontrolling electric charge in the surface region of the semiconductormaterial used as the starting substrate for the circuits. In the vastmajority of integrated circuits manufactured today, the semiconductorsubstrate is pure crystalline silicon.

Since a silicon crystal structure consists of a regular pattern ofsilicon atoms, its atomic arrangement can be described by specifyingsilicon atom positions in a repeating unit of the silicon lattice.

FIG. 1 shows the cubic unit "cell" of crystalline silicon, identifyingeight corner silicon atoms (C), eight exterior face silicon atoms (F)and four interior silicon atoms (I). It can be seen from FIG. 1 that ifa given silicon atom is considered as being at the center of a box, thenthat silicon atom is bonded to four neighboring silicon atoms thatdefine the four vertices of an equilateral pyramid. Five silicon atomsin the upper front corner of the FIG. 1 unit cell are accentuated toillustrate the basic building block. This building block may then beused to construct the cubic cell shown in FIG. 1, the so-called diamondcubic crystal structure.

The silicon diamond cubic crystal structure shown in FIG. 1 has theproperty that it may be repeated in three mutually perpendiculardirections to generate a silicon crystal of the desired size.

It also has the property that it may be represented by closely-packedplanes of atomic spheres stacked one on top of another to maximizedensity. A packing arrangement of this type results in the definition ofboth tetrahedral combinations of spheres and octahedral combinations ofspheres. A closely-packed, repetitive array of tetrahedra containsoctahedral interstitial spaces between the tetrahedra. Similarly, aclosely-packed, repetitive array of octahedral contains tetrahedralinterstitial spaces. Thus, a crystalline silicon lattice is said toinclude both tetrahedral and octahedral interstitial sites.

Control of electric charge in the surface region of crystalline siliconused for fabricating integrated circuits is achieved by introducingimpurity or "dopant" atoms into the silicon lattice. Depending on thedesired electrical characteristics, the dopant may be either "n-type" or"p-type."

When a dopant is introduced that has five valence electrons, i.e., onemore valence electron than silicon, an extra electron is provided thatdoes not fit into the bonding scheme of the silicon lattice. This extraelectron can be used to conduct current. The "n-type" dopant atoms (e.g.phosphorous (P), arsenic (As) and antimony (Sb)), are called donorsbecause, as Group V elements, they possess this extra electron. The "n"denotes negative and is used to represent the surplus of negative chargecarriers available in the silicon lattice with the dopant present.

When a dopant is introduced that has only three valance electrons, aplace exists in the silicon lattice for a fourth electron. This "p-type"dopant (such as boron) is called an acceptor. The "p" denotes positiveand represents the surplus of "holes", or positive charge carriers, thatexists in the lattice.

In the fabrication of integrated circuits, dopants are often introducedto the silicon lattice by diffusion. Diffusion is the mechanism by whichdifferent sets of particles confined to the same volume tend to spreadout and redistribute themselves evenly throughout the confined volume.In the case of integrated circuits, the diffusion process results inmovement and distribution of dopant atoms in the crystalline siliconlattice. In crystalline solids, diffusion is significant only atelevated temperatures where the thermal energy of the individual latticeatoms becomes great enough to overcome the interatomic forces that holdthe lattice together.

In crystalline silicon, dopants diffuse through the lattice by one oftwo diffusion mechanisms substitutional diffusion or interstitialdiffusion, or by a combination of the two. By substitutional diffusion,the dopant atoms move through the lattice by replacing a silicon atom ata given lattice site. By interstitial diffusion, the dopant atoms movevia the tetrahedral or octahedral interstitial sites in the latticestructure.

According to Fick's first law, particles tend to diffuse from a regionof high concentration to a region of lower concentration at a rateproportional to the concentration gradient between the two regions. Thiscan be mathematically expressed as: ##EQU1## where F is the net particleflux density, N is the number of particles per unit volume, and x is thedistance measured parallel to the direction of flow; D is the diffusioncoefficient, which is a property of the particular dopant and is anexponential function of temperature.

Fast diffusing dopant species, i.e., dopants having a high diffusioncoefficient, such as phosphorous (n-type) and boron (p-type), aredifficult to control within the crystalline silicon lattice. Thus, whenthese dopants are used, shallow diffusion region junctions and thelateral containment necessary for isolation of diffused dopant regionsare difficult to achieve.

Both Meyers et al, "Ge preamorphization of silicon: effects of dose andvery low temperature thermal treatments on extended defect formationduring subsequent SPE", Proc. Mat. Res. Soc., 52, 107 (1986), and Sadanaet al, "Germanium implantation into silicon: an alternativepreamorphization rapid thermal annealing procedure for shallow junctionformation", Proc. Mat. Res. Soc., 23, 303 (1984), have reported the useof germanium for preamorphizing silicon to reduce dopant diffusion.According to this technique, germanium atoms are introduced into thesilicon lattice to destroy crystallinity in the area of introduction.Then, the active dopant species is introduced into the preamorphizedarea. The structure is then subjected to a high temperature annealingprocedure which results in the creation of the desired dopant regions ina reconstructed crystalline lattice having germanium atoms atsubstitutional sites. The purpose of the preamorphization is to minimizedopant channeling during the creation of the dopant regions.

Germanium is chosen as the preamorphizing agent because of its unlimitedsolid-solubility in silicon. Furthermore, as a member of the same groupas silicon, germanium does not change the electronic configuration ofthe silicon lattice. It also has been found that the damage sites in thesilicon lattice introduced by germanium act as gettering centers.

While the germanium preamorphization procedure reduces the damage causedto the silicon lattice in the formation of diffused dopant regions, ithas little effect on the dopant diffusion control problems mentionedabove.

SUMMARY OF THE INVENTION

The present invention provides a method for inhibiting dopant diffusionin silicon using germanium. In accordance with the method, highconcentrations of germanium are introduced into substitutional sites ina silicon lattice. These germanium-rich substitutional regions attractinterstitial Group III or Group V dopants. Attraction for Group Vdopants continues when these n-type dopants are substitutional. GroupIII substitutional dopants, however, are repelled from regions of highgermanium concentration. This interactive behavior, along with the otherproperties of germanium in silicon, is used to create diffusion barriersuseful for both n-type and p-type dopant species.

A better understanding the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription of the invention and accompanying drawings which set forthan illustrative embodiment in which the principles of the invention areutilized.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a model silicon diamond cubic crystal structure.

FIG. 2 illustrates a model silicon lattice with substitutional germaniumatoms and defining two octahedral interstitial positions 01 and 02.

FIG. 3 illustrates a model silicon lattice with substitutional germaniumatoms and defining two tetrahedral interstitial positions T1 and T2.

FIG. 4 illustrates a model silicon lattice with substitutional germaniumatoms and defining two substitutional sites S1 and S2.

FIG. 5A is a plan view illustrating a crystalline silicon wafer.

FIG. 5B is a graph illustrating phosphorous concentration versus implantdepth in germanium-free areas of the silicon wafer.

FIG. 5C is a graph illustrating phosphorous concentration versus implantdepth in germanium-rich regions of the silicon wafer.

FIG. 5D is a graph illustrating phosphorous diffusion in germanium-freeareas of the silicon wafer.

FIG. 5E is a graph illustrating phosphorous diffusion in germanium-richregions of the silicon wafer.

FIGS. 6A and 6B schematically illustrate two applications of germaniumbarriers in silicon in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Our model calculations have established that high concentrations ofgermanium introduced into substitutional sites in crystalline siliconwill attract interstitial Group III (boron, aluminum) or Group V(arsenic, phosphorous) dopants. Attraction for the Group V dopantscontinues when these n-type dopants are substitutional. Group IIIsubstitutional dopants, however, are repelled from regions of highgermanium concentration.

FIG. 2 illustrates a model <100> silicon lattice with germanium atomsreplacing some of the silicon atoms. The large darkened circles in FIG.1 represent the germanium atoms, identified by the label "Ge". Allgermanium atoms in the FIG. 1 model occupy substitutional sites.Calculations were done assuming one of the two octahedral interstitialdopant positions, labelled "01" and "02" in FIG. 2, occupied by eitheran n-type (Group V) or a p-type (Group III) dopant atom. The "01"position is closer to the germanium-rich region. The calculated relativeenergies of dopants occupying the 01 and 02 interstitial positions areprovided in Table I below.

FIG. 3 shows a model <100> silicon lattice with substitutional germaniumatoms and defining two tetrahedral interstitial dopant positions,labelled "T1" and "T2". The "T1" position is closer to thegermanium-rich region. Calculations were done assuming one of thetetrahedral sites T1 or T2 occupied either by an n-type or a p-typedopant atom. The calculated relative energies of dopants occupying theT1 and T2 tetrahedral positions are provided in Table I below.

FIG. 4 shows a model <100> silicon lattice with substitutional germaniumand defining two substitutional dopant sites, labelled "S1" and "S2".Calculations were done with a n-type or a p-type dopant occupying one ofthese two sites. The calculated relative energies of dopants occupyingthe S1 and S2 substitutional sites are provided in Table I below.

A displacement of ±0.3 microns in the z-direction from a centralposition was used for purposes of the FIG. 2 octahedral dopant sitecomparison with dopants closer to or further away from thegermanium-rich region. In a pure silicon lattice, the displacementpositions would be equivalent in the bulk.

Similarly, the interactions of dopants at the FIG. 3 tetrahedralpositions would be identical in the bulk of a pure silicon lattice.

The FIG. 4 substitutional site S1 is closer to the germanium-rich regionthan site S2 and, where it is not, the difference in distance between itand site S2 is less than 20%. Again, the environments at S1 and S2 wouldbe identical in the bulk of a pure silicon lattice.

The total energies for the model systems illustrated in FIGS. 2-4 werecalculated using a self-consistent charge extended Huckle programdescribed in detail by S. Aronowitz, et al., "Quantum-chemical modellingof smectite clays", Inorg. Chem., 21, 3589-3593 (1982).

Table I presents the calculated difference in energies betweenconfigurations where the only change was the proximity of a dopant in aparticular site type (such as substitutional) to the germanium-richregion. If the energy difference is positive, i.e., ΔE=E₂ -E₁ >0, thenthe configuration E is more stable than the configuration E₂.

Commonly used n-type dopants, arsenic and phosphorous, along withcommonly used p-type dopants, aluminum and boron, were examined.

                  TABLE I                                                         ______________________________________                                               Relative Energies (eV).sup.a                                           Dopant   ΔE(O.sub.2 -O.sub.1).sup.b                                                          ΔE(T.sub.2 -T.sub.1).sup.c                                                          ΔE(S.sub.2 -S.sub.1).sup.d             ______________________________________                                        Arsenic  +25         +34         +46                                          Phosphorus                                                                             +24         +23         +18                                          Aluminum +16          +6         -25                                          Boron    +18         +14         -21                                          ______________________________________                                         .sup.a A position P.sub.2 is more stable than a position P.sup.1 if           ΔE < 0; otherwise, position P.sub.1 is more stable.                     .sup.b Positions O.sub.1 and O.sub.2 are displayed in FIG. 2. O.sub.1 is      closer to the germanium cluster.                                              .sup.c Positions T.sub.1 and T.sub.2 are displayed in FIG. 3. T.sub.1 is      closer to the germanium cluster.                                              .sup.d Positions S.sub.1 and S.sub.2 are displayed in FIG. 4. S.sub.1 is      closer to most of the germaniums comprising the germanium cluster.       

In all cases, when the dopant position species was interstitial(octahedral or tetrahedral), the more stable configuration was the onewhere the dopant was closer to the germanium-rich region. Thisuniformity was broken when the dopants were substitutional. In the casesinvolving substitutional dopants, configurations involving n-typedopants showed greater stability when the dopants were closer to thegermanium-rich region, whereas configurations involving p-type dopantsshowed greater stability when the p-type dopants were in substitutionalsites further from the germanium-rich region.

Consequently, n-type dopants in regions with high germaniumconcentration, whether interstitial or substitutional, tend to remain inthose regions; dopant diffusion from such regions is markedly reduced.

This behavior has been observed experimentally for phosphorous.

The "phosphorous" experiment will be described with reference to FIGS.5A-5E.

FIG. 5A shows a plan view of a crystalline silicon wafer 10. The innercircular area 12 of the wafer 10 consists of dark regions 14 which havebeen masked by photoresist and light regions 16 which have been leftexposed.

Germanium is implanted into the light areas 16 at an implant dosage of1.5×10¹⁶ Ge/cm² at an implant energy of 80 keV. The photoresist was thenstripped and the wafer was annealed in nitrogen at 1000° C. for 30minutes to drive the germanium atoms to substitutional sites within thecrystalline silicon lattice.

Following the substitution of the germanium atoms into the siliconlattice, the region 12 was implanted with phosphorous at an implantdosage of 1.5×10¹⁵ P/cm² and an implant energy of 30 keV. FIG. 5B showsthe concentration profile of phosphorous in the germanium-free (dark)areas, that is, those areas that had been masked by photoresist strips14. FIG. 5C shows the phosphorous profile in the germanium-rich regions16 of area 12.

Following the phosphorous implant, the wafer 10 was annealed in nitrogenat 900° C. for 30 minutes. A comparison of FIGS. 5D and 5E will show,respectively, that the junction depths for the germanium-free regionsare deeper than the junction depth for the germanium-rich regions. Theseresults are consistent with the calculated finding that regions ofsubstitutional germanium will inhibit diffusion of n-type dopants.

The behavior of p-type dopants in regions of high germaniumconcentration depends on whether the dopants are at interstitial orsubstitutional sites in the lattice. When interstitial, the diffusion ofp-type dopants is significantly reduced; however, upon enteringsubstitutional sites, the p-type dopants exhibit enhanced diffusion fromthe high concentration germanium region.

The above-described attraction to or repulsion from regions of highsubstitutional germanium concentration can be used to devise suitablediffusion barriers.

These findings may be applied for p-type dopants by creating two regionsin a silicon lattice where germanium in high concentration exists. Thegermanium-to-silicon ratio in the peak concentration areas of thegermanium-rich regions of the silicon lattice should be greater than orequal to Ge:Si=1:5000 and less than Ge:Si=1:10. The results presented inthis discussion are based on a ratio of Ge:Si≃1:10.

FIG. 6A shows the germanium barrier used to control junction depth.Germanium is implanted at two different energies. First, germanium at adosage of 10¹⁶ /cm² is implanted at low energy (30-80 keV) to provide afirst germanium-rich region I. Then, germanium at a dosage of 10¹⁶ /cm²is implanted at high energy (>180 keV) to provide a secondgermanium-rich region II. The lower implant energy is chosen so that thepeak concentration of germanium in region I will be at least equal indepth to the peak concentration of the p-type dopant species when it isintroduced. The higher implant energy is chosen to produce a distinctsecond peak in region II that will be the junction boundary. A hightemperature anneal follows the double germanium implant to drive thegermanium to substitutional sites in the silicon crystal structure andprecedes any p-type dopant implant into that region.

FIG. 6B shows schematically the use of germanium barriers to reducelateral dopant diffusion into the vicinity of a trench 10. The two setsof double germanium implants, performed as described above with respectto the FIG. 6A junction depth example, are designed to control both nearinterfacial lateral diffusion as well as deeper lateral or bulkdiffusion to the interface. A high temperature anneal followsimplantation of the double germanium sets. The anneal precedes anyadditional dopant implants into that region. The dotted curve on theright hand side of FIG. 6B represents a future p+ region.

Since, as stated above, n-type dopants tend to remain in thegermanium-rich regions of the lattice, formation of a singlegermanium-rich region is sufficient to inhibit n-type dopant diffusion.This region is created by introducing germanium at an implant energychosen so that the peak concentration of germanium will be at leastequal in depth to the peak concentration of the n-type dopant specieswhen it is introduced.

In summary, germanium, when distributed in substitutional sites insilicon so as to form regions of high concentration regions that areinterposed between the region where dopants are introduced and theregion where these dopants are to be excluded, will act as a diffusionbarrier to p-type dopants. In the case of n-type dopants, a singlegermanium-rich region is sufficient to inhibit diffusion.

In accordance with the present invention, germanium, once implanted intosilicon, is annealed to remove damage and to drive the germanium tosubstitutional sites before an electrically-active dopant species isintroduced.

This approach permits formation of very shallow junctions with eithern-type or p-type dopants. This is useful for bothmetal-oxide-semiconductor (MOS) and bipolar devices. It permits sharplydefined dopant distributions and channel regions for MOS devices as wellas offering isolation barriers with respect to lateral diffusion.

It should be understood that various alternatives to the embodiments ofthe invention described herein may be employed in practicing theinvention. It is intended that the following claims define the scope ofthe invention in that structures and methods within the scope of theseclaims and their equivalents be covered thereby.

What is claimed is:
 1. A semiconductor structure for controlling lateraldiffusion of electrically-active dopant atoms in a crystalline siliconlattice, the structure comprising:(a) first and second verticallyspaced-apart germanium-rich regions of the silicon lattice havinggermanium atoms at substitutional sites in the silicon lattice, thefirst and second germanium-rich regions being substantially verticallyaligned in the silicon lattice; and (b) third and fourth verticallyspaced-apart germanium-rich regions of the silicon lattice havinggermanium atoms at substitutional sites in the silicon lattice, thethird and fourth germanium-rich regions being substantially verticallyaligned in the silicon lattice, wherein the first and secondgermanium-rich regions are laterally spaced-apart from the third andfourth germanium-rich regions by a region of the silicon lattice intowhich P-type dopant atoms are to be introduced.
 2. A semiconductorstructure as in claim 1 wherein the germanium-to-silicon ratio in thepeak concentration area of the first, second, third and fourthgermanium-rich regions is in the range of 1:5000 to 1:10.